Sleep

Why is it sometimes you feel like you slept for hours when it's only a couple hours and then sometimes it feels like you haven't slept at all when you slept a full 8 hours?
 
I can sleep 7-8 hours one night and still feel tired, then the next night I can sleep the same amount but feel amazingly rested.

I don't get it.

Waking Up

Why is it that I have no trouble waking up early on weekends or on vacation? But, if I have to wake up for work, I feel like I can't open my eyes and my body feels like a ton of bricks...